As is well known in the art, significant advances have been made in telecommunications systems over recent years, particularly in the rate at which information can be communicated. Modern digital telecommunications systems and communication media provide very high bandwidth, such as the 44.736 Mbps data rate provided by the DS-3 data frame standard. Furthermore, conventional fiber optic cable and systems can provide even higher bandwidth and data rates by time-division multiplexing of up to twelve DS-3 lines, providing bandwidth of up to 536.8 Mbps.
These extremely high bandwidths now available in digital telecommunications systems have enabled the communication of large volumes of data at high speeds. Since voice channels require very little bandwidth (on the order of 4 kbps each), a large number of voice channels may now be communicated over a single communication line by way of time division multiplexing. The available bandwidth now also enables the communication of large blocks of digital data from computer-to-computer, as well as digital data representative of other media such as video displays.
Unlike voice transmissions, however, in which some amount of errored signals can be readily tolerated without garbling of the message, the successful transmission of digital data among computers requires high reliability and high quality transmission. Accordingly, conventional digital cross-connects now provide "performance monitoring" (commonly referred to as "PM"), by which the error rate of received digital data is monitored by way of cyclic redundancy check (CRC) and other conventional coding techniques. Such performance monitoring is used to ensure the desired grade of service desired by those telecommunications customers paying premium tariffs for high quality and low error rate communications.
Conventional telecommunications systems also generally provide some amount of redundancy so that failure of a telecommunications line or network element does not result in the loss of the communicated message. Conventional telecommunications systems with performance monitoring have implemented certain alarm conditions by which a human operator is alerted to events such as "loss of signal" and to error rates exceeding various thresholds. In response, the operator can manually switch to a redundant line to again enable communication of the digital data in the system. Of course, the procedure of generating an alarm condition and the manual switching of input/output ports to other lines cannot be effected quickly.
By way of further background, conventional fiber optic terminals (commonly referred to as FOTs) have implemented 1:1 redundancy for the fiber optic lines in a system, with some amount of automatic switching. According to this 1:1 redundancy scheme, the overhead portion of the bandwidth is monitored to determine if a loss-of-signal ("LOS") or alarm indication signal ("AIS") condition is being received. In these FOT 1:1 redundancy schemes, upon receipt of an LOS or AIS signal, the FOT will automatically switch its transmission to the other of the two fiber optic lines, enabling transmission of the data despite the failure of the first fiber optic line.
In addition, other conventional telecommunications systems include 1:1 redundancy schemes. However, since such protection schemes require that both the cabling and the input/output ports from cross-connects must be doubled from that required to carry traffic, such protection schemes are quite expensive and are thus available to customers only at high premium rates. As a result, the implementation of such protection schemes is generally limited to high grade of service communication, such as for the telecommunication of computer data.
By way of further background, conventional fiber optic systems operating according to the SONET standard can include 1:n line protection, in which one of several fiber optic lines in a group can be switched to a protection line. Such a protection scheme is described in the SONET specification TA-NWT-000253, Issue 6 (BELLCORE, September 1990). As noted above in the 1:1 fiber optic case, this protection is provided based on line data, with entire lines switching to the protection channel; no protection is provided on a path-by-path basis. Furthermore, the communication between nodes according to the SONET 1:n line protection scheme is by way of the K1, K2 bytes in the line overhead data. However, the use of such protocol is not available for providing 1:n protection of any kind, especially path protection, to a DS-3 network, due to insufficient available bandwidth.
It is therefore an object of the invention to provide a method and system for providing facility protection for a group of paths in a digital telecommunications system, where one redundant channel and port can protect multiple member ports.
It is a further object of the present invention to provide such a method and system which is useful in DS-3 bandwidth communications systems.
It is a further object of the present invention to provide such a method and system which effects such protection utilizing available bandwidth in a DS-3 standard system.
It is a further object of the present invention to provide such a method and system to provide such protection where the switching time between the member and protection channels for a failing path is extremely short, such as on the order of 100 msec or less.
It is a further object of the present invention to provide such a method and system which is revertive, in that the protection path is freed upon the member path returning to a good signal condition.
It is a further object of the present invention to provide such a method and system in a distributed manner in a digital cross-connect.
Other objects and advantages of the present invention will become apparent to those of ordinary skill in the art having reference to the following specification together with the drawings.